1. Field of the Invention
This invention relates to cooling towers and particularly to a molded, composite airfoil-defining, synthetic resin blade for large diameter cooling tower fans and having an abrasion resistant leading edge.
2. Description of the Prior Art
Industrial size induced draft water cooling towers have one or more relatively large diameter fans which pull in air from the surrounding atmosphere and direct such air through the water to be cooled by evaporative effect, before discharge of the hot air through a velocity recovery stack. Fans for these applications generally are of a diameter within the range of from about 12 feet to as much as 60 feet or more.
Small diameter cooling fans within the range of from 2 feet to 12 feet in diameter have for the most part been made of metal such as aluminum. Large diameter industrial cooling tower fans having diameters of from 12 feet to as much as 60 feet on the other hand have often been manufactured from fiberglass reinforced synthetic resin in order to reduce the overall weight of the blade and hub assembly. In small diameters, cooling fan blades of aluminum are less expensive than plastic blades. However, for industrial size fan blades, design constraints often preclude the use of aluminum or other metals. Plastics, usually reinforced with materials such as fiberglass, are the construction materials of choice. Aluminum blades for example become too heavy where the blades are to be used in fans having a diameter of 20 feet or more.
Plastic fan blades made up of synthetic resin material reinforced with glass fibers have for the most part been manufactured of an epoxy resin containing fiberglass reinforcement. However, the cost of the resin and the limitations on the use of thermoset type resins such as epoxies, have made epoxy blades very expensive to manufacture and difficult to sell with a reasonable return on the investment.
Polyester fan blades, on the other hand, are less expensive because of the lower price resin, but it has not been heretofore feasible to fabricate polyester having physical and chemical properties commensurate with those of epoxies. Abrasion and consequent deterioration of the leading edge of polyester fan blades has been a particularly vexatious problem.
A need thus exists for a reasonably priced plastic blade for large diameter industrial water cooling tower fan applications manufactured of a polyester resin or the like where the leading edge exhibits adequate abrasion resistance and does not rapidly deteriorate in use while still retaining a requisite surface finish, necessary strength characteristics, required compound curve configuration, adequate strength to weight ratios, and required longevity. Heretofore, these requisites have not been obtainable at a competitive price.
Composite aircraft propellers manufactured of synthetic resin reinforced with glass fiber material and formed over foam cores have been available for a number of years but the problems presented in the manufacture of aircraft blades are significantly different from those encountered in the design and fabrication of significantly longer blades used in industrial cooling towers. Aircraft propellers of plastic materials have relied upon metal leading edge covers of nickel or stainless steel, utilizing technology that has long been practiced in connection with the manufacture of wooden blades. Examples of composite aircraft blades with metal leading edges are illustrated and described in Hartzell Propeller, Inc., U.S. Pat. Nos. 4,102,155 and 4,810,167. Aircraft propellers though sell for a significantly higher cost on a linear basis than can be charged for industrial water cooling tower fans and thus it is not commercially practical to employ the technology that has been developed and is in use for manufacture of water cooling tower fan blades.